New Delhi: India's planned Rs 40,000-crore deal to buy the sophisticated, long-range S-400 air defence system from Russia would "complicate" building inter-operability between the United States (US) and Indian militaries, a senior US Congressman said in Delhi on Monday.

The panel oversees the Pentagon, all military services and all departments of defence agencies, including their budgets and policies. Thornberry and three other members of the panel were in Delhi enroute to the 17th Asia Security Summit (Shangri La Dialogue 2018) in Singapore. He said the US was making sanctions against Russia "flexible" for its allies like India to allow them breathing room to continue dealing with Moscow only to maintain their legacy equipment but at the sametime, they must wean themselves off Russian equipment."There is a lot of concern in the US in both the administration and the Congress regarding the S-400 (missile) system. And there is concern that any country, and it is not just India that is looking at clearing it, but any country that acquires that system will complicate our ability to work out inter-operability," Republican Mac Thornberry, the chairman of the House Armed Services, told a group of journalists in Delhi. "So that is completely apart from any sanctions, legislation. I hope the government (of India) will take its time and consider very carefully the acquiring of that system because the difficulties it may pose for us."

The US sanctions against Russian oligarchs and companies, including Rosoboronexport, the state-owned Russian weapons trading company, has raised concerns in India about a possible impact on India's military buys from Moscow, particularly the deal to procure S-400 Triumf long-range surface-to-air missile systems. The missile system is billed as a game changer by the Indian military for its ability to counter ballistic missiles and stealth aircraft like those China is developing. India and Russia signed an agreement in principle for the S-400 deal in 2016.

India's deep military and strategic ties with Russia date back to the beginning of the Cold War even as New Delhi led a movement of "non-aligned" countries that declared their tilt with neither Washington nor Moscow. However, India always leaned toward the USSR.

India still buys over 60 percent of its defence equipment from Russia. At present, the Indian armed forces are 70 percent equipped with Soviet or Russian weapons.

Thornberry said a provision in the National Defense Authorization Act (NDAA), which is awaiting a Senate nod and was passed by the House on 24 May, would allow the Trump administration to suspend the sanctions under the Countering America's Adversaries Through Sanctions Act, or CAATSA.

But the President, in order to seek the waiver, has to make a certification to Congress that the country subject to sanctions and seeking relaxation is altering its relationship with Russia.

"The current sanctions bill that was just signed in the law last fall doesn't really have much flexibility. But the bill that passed by the House on Thursday adds more flexibility for nations like India who have a legacy of Russian military equipment and of course need to purchase spare parts in order to maintain the readiness of that equipment.

"There is an understanding both in the administration and in Congress that some additional flexibility in that law is needed and I believe that there will be...we have to take our bill and reconcile it with the Senate version. But I think there will be some additional flexibility that is... It is not just India there are some other nations as well."

He said the flexibility at this point allowed the Secretary of Defense some discussion if a country was reducing its dependency on Russia and "has a desire to continue to do so". "Some of the government officials (in India) believe that the language could be improved. I am certainly willing to hear suggestions from not only the Indian government but other governments only," Thornberry said.

Asked how the US viewed India's dependency on Russia military hardware, he said America would want to encourage New Delhi's efforts in "seeking to diversify its suppliers of military equipment". "It will be a topic of ongoing conversation about whether that is going at an appropriate pace and it makes sense from an Indian point of view that it wants to be less dependent on one particular supplier given the concerns on what this particular supplier has been doing on the world stage."

He said the House panel members were in India to affirm the strong bipartisan support in Congress to the India-US strategic partnership. "We in Congress can contribute in enhancing that partnership. We have had some good and productive discussions with people in government and some people outside the government."